During certain semiconductor assembly processes, semiconductor dice are placed on a carrier such as a leadframe substrate. Electrical connections in the form of wire bonds are then made between the dice and substrate, or between individual dice. Gold, aluminium or copper wires are commonly used to make these connections. Wire bonds are formed at bonding sites where the electrical connections are to be made, typically using an ultrasonic transducer to generate ultrasonic energy to attach a length of wire fed from a capillary to the semiconductor device or carrier. After these wire bonds are made, the dice, wire loops and substrate are encapsulated with a resin material to protect the same. A semiconductor package is produced.
There is a continuing desire in the semiconductor industry to develop ever smaller and thinner semiconductor packages. Since, as explained above, the wire loops should be fully encapsulated in the final package, the thickness of the package would be affected by the heights of the wire loops that are formed during wire bonding. If the heights of the wire loops can be kept to a minimum, then the thickness of the final package can be correspondingly reduced.
Furthermore, there is a demand in the industry for semiconductor devices with stacked dice. The advantage of having stacked dice is that stacked dice incorporate more silicon functionality by stacking multiple dice into a single package. This reduces overall size by eliminating additional packages on the circuit board. Furthermore, it increases space savings while enhancing electrical performance by reducing propagation time for signals to traverse from one chip to another. Stacked dice allow a greater density of integrated circuits on a given area of the carrier, and may increase efficiency. Since each die in the stack would require an electrical connection to the carrier, or to another die, several layers of wire loops are formed. Correspondingly, it would be better for wire loops to be profiled as low as possible to cater for this need.
Thus, there is a desire in the semiconductor industry to seek to address this need to form wire loops with low height profiles. For example, U.S. Pat. No. 5,156,323 (“Wire Bonding Method”) seeks to form a low wire loop by performing successive reverse action motions after making a first bond by moving a capillary away from the second bonding point before feeding sufficient wire to make a wire loop to connect the first bond to a second bonding point. Another example is U.S. Pat. No. 5,961,029 (“Wire Bonding Method”) which also performs successive reverse action motions over a first bonding point, then forming a kink in an inclined portion of a trapezoidal wire loop before connecting the first bond to a second bonding point.
FIG. 1(a) is a side view of a conventional loop motion profile of a prior art wire bonding device employing reverse action motion. A bond is formed with wire fed out of a capillary at a first bonding point A. The capillary is then moved up by a certain distance to point B. Thereafter, the capillary is moved in a direction away from the second bonding point G in a reverse action motion to point C. The capillary is moved up by a further distance to point D, before the capillary executes another reverse action motion to point E. The capillary is then moved diagonally upwards to point F in a forward motion with such an amount of wire fed out as is necessary for forming a wire loop to a second bonding point G. Finally, the capillary is moved to the second bonding point G whereat a second bond is made.
FIG. 1(b) is a top view of the conventional loop motion profile of the said prior art wire bonding device. The motion of the capillary from the first bonding point A through to second bonding point G essentially forms a straight line from a top view of the capillary.
Using the aforesaid conventional bonding method, the ability to decrease the loop height of the wire is limited by the weakness caused to the neck of the wire at the point where the wire is bent towards the second bonding point, and the heat to which the wire is subjected. Such weakness in the neck of the wire is caused by the reverse motion of the capillary followed by pulling the wire in an opposite direction by the forward motion. If the loop height is less than 2.5 times of the wire diameter, the neck of the loop is at risk from cracking, making the resulting connection unreliable or unstable. Furthermore, if the neck is weakened, subsequent pulling of the wire during the process of forming the second bond may also potentially break the wire.